Using resource bundle in <s:actionmessage/> tag
In your theme, override actionmessage.ftl with
<#if (actionMessages?? && actionMessages?size > 0)>
<ul<#rt/>
<#if parameters.cssClass??>
class="${parameters.cssClass?html}"<#rt/>
<#else>
class="actionMessage"<#rt/>
</#if>
<#if parameters.cssStyle??>
style="${parameters.cssStyle?html}"<#rt/>
</#if> >
<#list actionMessages as message>
<li><span><@s.text name="${message!}"/></span></li>
</#list>
</ul>
</#if>